style: Factor out adding a rule in CascadeData::add_rule

This shouldn't have any behavior change, but is necessary because for
cascade layers we are going to need to handle the child rules / sheets
ourselves, in order to handle nested layers properly.

Differential Revision: https://phabricator.services.mozilla.com/D124334

components/style/stylist.rs

@@ -2125,10 +2125,11 @@ impl CascadeData {
         }
     }
 
-    // Returns Err(..) to signify OOM
+    #[inline]
-    fn add_stylesheet<S>(
+    fn add_rule<S>(
         &mut self,
-        device: &Device,
+        rule: &CssRule,
+        _device: &Device,
         quirks_mode: QuirksMode,
         stylesheet: &S,
         guard: &SharedRwLockReadGuard,
@@ -2138,18 +2139,6 @@ impl CascadeData {
     where
         S: StylesheetInDocument + 'static,
     {
-        if !stylesheet.enabled() || !stylesheet.is_effective_for_device(device, guard) {
-            return Ok(());
-        }
-
-        let contents = stylesheet.contents();
-        let origin = contents.origin;
-
-        if rebuild_kind.should_rebuild_invalidation() {
-            self.effective_media_query_results.saw_effective(contents);
-        }
-
-        for rule in stylesheet.effective_rules(device, guard) {
         match *rule {
             CssRule::Style(ref locked) => {
                 let style_rule = locked.read_with(&guard);
@@ -2162,7 +2151,7 @@ impl CascadeData {
                     if let Some(pseudo) = pseudo_element {
                         if pseudo.is_precomputed() {
                             debug_assert!(selector.is_universal());
-                                debug_assert!(matches!(origin, Origin::UserAgent));
+                            debug_assert_eq!(stylesheet.contents().origin, Origin::UserAgent);
 
                             precomputed_pseudo_element_decls
                                 .as_mut()
@@ -2306,6 +2295,43 @@ impl CascadeData {
             // We don't care about any other rule.
             _ => {},
         }
+        Ok(())
+    }
+
+    // Returns Err(..) to signify OOM
+    fn add_stylesheet<S>(
+        &mut self,
+        device: &Device,
+        quirks_mode: QuirksMode,
+        stylesheet: &S,
+        guard: &SharedRwLockReadGuard,
+        rebuild_kind: SheetRebuildKind,
+        mut precomputed_pseudo_element_decls: Option<&mut PrecomputedPseudoElementDeclarations>,
+    ) -> Result<(), FailedAllocationError>
+    where
+        S: StylesheetInDocument + 'static,
+    {
+        if !stylesheet.enabled() || !stylesheet.is_effective_for_device(device, guard) {
+            return Ok(());
+        }
+
+        let contents = stylesheet.contents();
+
+        if rebuild_kind.should_rebuild_invalidation() {
+            self.effective_media_query_results.saw_effective(contents);
+        }
+
+
+        for rule in stylesheet.effective_rules(device, guard) {
+            self.add_rule(
+                rule,
+                device,
+                quirks_mode,
+                stylesheet,
+                guard,
+                rebuild_kind,
+                precomputed_pseudo_element_decls.as_deref_mut(),
+            )?;
         }
 
         Ok(())